1. Field of the Invention
The present invention relates to a method of tone reproduction with halftone dots, an apparatus for outputting a halftone plate, a halftone plate, and a printed material, which are suitable for application to a printing press, a CTP (Computer To Plate) system, a CTC (Computer To Cylinder) system, a DDCP (Direct Digital Color Proofing) system, a film setter, etc. which are halftone dot image outputting apparatus for generating a multicolor-separated halftone dot image in the graphic arts fields, to prevent tone jumps in halftone dot images.
2. Description of the Related Art
In recent years, there have widely be used in the print platemaking field image scanning reading and recording apparatus for electrically processing the image information of a subject to produce a film plate for printing in order to make the printing operation efficient and improve the quality of images.
The image scanning reading and recording apparatus are basically constructed of an input section, a control section, and an output section. In the input section, an image signal is captured from a subject by an illuminating optical system, a color-separating optical system, and a photometric system.
The image signal which has been photoelectrically converted by the input section is then processed by the control section for tone correction, color correction, edge emphasis, and signal conversion from R (Red), G (Green), and Blue (B) signals into C (Cyan), M (Magenta), Y (Yellow), and K (Black) signals.
The image signal which has been processed by the control section is converted by the output section into an optical signal such as a laser signal or the like, which is processed to record a signal on a recording medium that is made of a photosensitive or thermal material. The image recorded on the recording medium is developed, and the recording medium is then used for printing.
If a subject to be duplicated by printing represents a continuous-tone image such as a photograph, a painting, etc., then it is necessary to screen the subject so as to reproduce the tone of the image with halftone dots. Specifically, on a film plate, the continuous-tone image of the subject is converted to a halftone dot image that is made up of a collection of halftone dots whose sizes depend on the highlight and shadow areas of the continuous-tone image.
It is known in the art that there are available halftone dots of different shapes, e.g., circular dots, quadrilateral (square or rhomboidal) dots, dots of geometrically mixed shapes, etc., as described in pages 68, 69, and 70 of “Postscript screening” 1st Edition, 1st Printing, written by Peter Fink, published by MDN Corporation, Aug. 11, 1994.
FIG. 9 of the accompanying drawings shows a gradation scale 2 of rhomboidal dots whose halftone percentage continuously grows from 0% to 100%.
FIG. 10 of the accompanying drawings shows a gradation scale 4 of circular dots whose halftone percentage continuously grows from 0% to 100%.
FIG. 11 of the accompanying drawings shows a gradation scale 6 of dots of geometrically mixed shapes whose halftone percentage continuously grows from 0% to 100%.
In order to form images of halftone dots, it is preferable to generate halftone plates such as film plates capable of a smoother tone reproduction free of tone jumps.
In the gradation scale 2 shown in FIG. 9, square halftone dots (including rhomboidal halftone dots) range from a highlight area which is a bright area of lower density through an intermediate tone area to a shadow area which is a dark area of high density. It is known in the art that the tone reproduction of the shapes of the respective dots in the gradation scale 2 cannot be accurately performed in both the highlight area and the shadow area.
In the gradation scale 4 shown in FIG. 10, circular halftone dots range from a highlight area of lower density through an intermediate tone area to a shadow area of high density. It is known in the art that the tone reproduction of the shapes of the respective dots in the gradation scale 4 can be accurately performed in the highlight area, but it cannot be accurately performed in the shadow area, and a tone jump occurs at halftone dot junctions in the vicinity of the halftone percentage of 78.5%.
In the gradation scale 6 shown in FIG. 11, a highlight area is composed of circular halftone dots, an intermediate tone area at the halftone percentage of 50% is composed of square halftone dots, and a shadow area is composed of dots that are in symmetrically shaped relation to the dots in the highlight area across the halftone percentage of 50%. In the gradation scale 6, the tone reproduction of the shapes of the respective dots can be accurately performed in both the highlight area and the shadow area, and the gradation changes smoothly up to the halftone percentage of 50%. However, the gradation scale 6 made up of dots of geometrically mixed shapes is disadvantageous in that a large tone jump occurs in the vicinity of the halftone percentage of 50%.